There is going to be a dormer added this summer as planned, and I have some questions about how insulation is typically installed on the new side walls of the dormer.
Like this picture seems to be most common, meaning the insulation is extended outward and doesn’t take up any additional space inside the dormer:
Or like this dormer where the framing is inward
The first option is best if you want to make use of all the space you can from the dormer, but it must be worse insulated? Meaning it easily sneaks through between the roof/wall of the dormer?
Maybe it’s common to have a 140mm frame externally and then add 70mm internally or something similar to counteract this?
I'm not entirely sure what you mean by "building outward" versus "building inward," but I interpret it as wanting the dormer's internal width to be as large as the placement of the existing rafters allows. The dormer's side walls should ideally not infringe on this width. There is no technical obstacle to making the side walls as wide as normal insulation standards require, but it can result in some aesthetic (and unwanted) effects. Often, it may be easier to compensate for the lack of wall insulation with thicker insulation in the roof. Consider the dormer's roof shape a bit more carefully. Weigh both what is customary in the area and how your house looks in cross-section. It's always nicest when the dormer's interior ceiling is a horizontal extension of the attic's.
I'm not entirely sure what you mean by "building outward" versus "building inward," but I interpret it as you wanting the dormer's internal width to be as large as possible based on the existing rafters' placement. The dormer's side walls should preferably not encroach on this width. There is no technical obstacle to making the sidewalls as wide as normal insulation standards require, but it could result in some aesthetic (and undesired) effects. Many times, it might be easier to compensate for the lack of wall insulation with thicker insulation in the roof. Consider the dormer's roof shape carefully. Weigh both what is customary in the area and how your house looks in cross-section. It's always nicest when the dormer's ceiling is a horizontal extension of the attic's.
the roof's insulation will be the same as the rest, including height, but yes, you understood correctly with "building outward" and "building inward."
but I added an arrow in the first picture, and when filling the framework with insulation, it will be poorly insulated against the existing roof, especially when placing the "syllen" (?) on the dormer's side walls over the existing roof's battens?
it's possible to remove the battens and lower the dormer's outer wall directly against the rafter of the truss, but it will still be poorly insulated with the air gap on the existing roof?
maybe it's worth it compared to losing space inside the dormer?
perhaps a good compromise is to build up the dormer as shown in the picture but also internally build out with 45x70 or something to minimize cold drafts where the arrow points?
another picture where the wall is built outward and where I think there will be poor insulation, cold drafts?
No, you can’t do that, embedding the load-bearing battens into the dormer’s framework. The slanted sill for the dormer wall should lie directly on the truss’s rafter, or the roof deck if there is one? Upon closer inspection of the pictures, it seems that the underlay is some form of board, that the load-bearing battens are a sturdy beam about 45x70 mm, and that the furring strips are missing. It's not an entirely uncommon construction, but not optimal when building a dormer. You need to construct some form of support for the ends of the load-bearing battens outside the dormer wall, otherwise, you can’t replace the battens without partially dismantling the dormer. I’m a bit doubtful about the builders’ competence.
No, you can't do that, integrate the support battens into the dormer's framework. The slanted base plate of the dormer's wall should lie directly on the roof truss's rafter, or on the underlayment's raw boards, if there are any? When I look closer at the pictures, I think I detect that the underlayment is some kind of board, that the support batten is a robust beam about 45x70 mm and that there is no counter batten. It's a not unusual construction, but not optimal when you want to build a dormer. You'll need to create some kind of support for the ends of the support batten that lie outside the dormer wall, otherwise you can't replace the support batten without partially tearing down the dormer. I'm a bit skeptical about the builders' competence.
I also thought it felt wrong to integrate the support battens in that way.
I have the same roof construction you described, masonite board with 45x70 support battens, so it will be necessary not to build the wall "outwards" from the roof truss but more like this picture:
And with double rafters so the existing roof truss holds the support battens steadily and is easy to replace in the future. The only downside is that you get less space this way, but it is probably the best solution, especially with roofs lacking raw boards.
It will be difficult/impossible to get this metal sheet (2) in place if you build like the first picture, right?:
Yes, that sheet is important to prevent water splashing under the tiles. It must be expected that the battens with this construction (as with all battens, for that matter) will not last forever. They will become moisture damaged where they are nailed to the rafters. Concrete tiles have a realistic lifespan of 40 years. Before then, they may start to leak due to moss and other factors.
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